Vitor, do take a look at unifiedpush (https://unifiedpush.org/). It's a very developer friendly, cross platform framework that lets you do decentralized push notifications outside of the grasp of tech titans. It has zero dependence on FCM or Apple services. It works amazingly well on Android with ntfy (https://github.com/binwiederhier/ntfy-android) which can be either hosted or self-hosted.

Here's a simple android unifiedpush integration example in their github: https://github.com/UnifiedPush/android-example